1. Field of the Invention
This invention relates in general to directional couplers and in particularly to a directional coupler formed in strip line technique.
2. Description of the Prior Art
Directional couplers formed in strip line technology are known which consist of two strip lines applied to a surface of a dielectric carrier and extend parallel to each other which strip lines form two microstrip lines with the grounded backside of the dielectric supporting plate. Electrical and magnetic coupling exists between these two microstrip lines. Such a directional coupler arrangement in microstrip technology is described in the article by Bryant, Weiss in IEEE Transactions MTT-16, pages 1021-1027 of December 1968. However, such device has very poor sharpness of directivity particularly at high coupling attenuations.
So as to improve the sharpness of directivity which assures better confirmation of the velocity in the in-phase and push-pull waves, there are a number of possible solutions. For example, the article by H. J. Herzog in the Electronics Letters, Volume 14, page 50, No. 3, Feb. 2, 1978 discloses loading all four coupling points of the microstrip directional coupler by means of parallel capacitances. Another possible embodiment of a microstrip directional coupler is disclosed in the article by G. Schaller in AEU Volume 26, No. 11, 1972, pages 508-509 wherein a capacitive coupling between the two coupled lines exist at their outputs. The design of the coupling section has a graduation of directional couplers with different coupling attenuations and different lengths is discussed in the article by S. Rehnmark, IEEE Transactions MTT-25, pages 1116- 1121 of December, 1977. A sawtooth shaped arrangement for the coupling section is described in the article by A. Podell, 1970 G-MTT, International Microwave Symposium Digest. The application of overlays on the coupling section is described in an article by B. Sheleg and B. E. Spielman in IEEE Transactions MTT-22, pages 1216-1220 of December 1974. The employment of special anisotropic substrate material is disclosed in the article by N. G. Alexopoulos, C. M. Krowne in the periodical IEEE Transactions MTT-26, pages 387-393 of June 1978. An arrangement of a slit on the grounded side of two coupled microstrip lines is disclosed in the article by M. Aikawa in Transactions of the IECE of Japan, Volume E. 60, No. 4, pages 206-207 of April 1977.
In all of these known directional coupler arrangements in microstrip technology, the demands for a high electrical strength, a high coupling attenuation and at the same time for independence of sharpness of directivity for various housing dimensions are not satisfactorily met.
A directional coupler formed in strip line techniques is described in German A.S. No. 21 02 554 wherein one transmission line is designed as a microstrip line and the other transmission line is designed as a slit line. This strip line is formed by means of a narrow gap dividing the grounded conductor surface of the microstrip line with the gap arranged opposite the strip line of the microstrip line in such a manner that the longitudinal radii of the strip line and of the gap enclose a small angle of crossing which differs from zero. In this structure, however, difficulties in the series connection of components and upon transition to a microstrip line are encountered. Also, in such a slit line embodiment of a directional coupler, a significant leakage field occurs. Transmission down to the zero frequency is not possible with this arrangement.